Bottom Line:
Nonradiative relaxation of high-energy excited states to the lowest excited state in chlorophylls marks the first step in the process of photosynthesis.Modeling this process with non-adiabatic excited state molecular dynamics simulations uncovers a critical role played by the different side groups in the two molecules in governing the intramolecular redistribution of excited state wavefunction, leading, in turn, to different time-scales.This is achieved via selective participation of specific atomic groups and complex global migration of the wavefunction from the outer to inner ring, which may have important implications for biological light-harvesting function.

ABSTRACTNonradiative relaxation of high-energy excited states to the lowest excited state in chlorophylls marks the first step in the process of photosynthesis. We perform ultrafast transient absorption spectroscopy measurements, that reveal this internal conversion dynamics to be slightly slower in chlorophyll B than in chlorophyll A. Modeling this process with non-adiabatic excited state molecular dynamics simulations uncovers a critical role played by the different side groups in the two molecules in governing the intramolecular redistribution of excited state wavefunction, leading, in turn, to different time-scales. Even given smaller electron-vibrational couplings compared to common organic conjugated chromophores, these molecules are able to efficiently dissipate about 1 eV of electronic energy into heat on the timescale of around 200 fs. This is achieved via selective participation of specific atomic groups and complex global migration of the wavefunction from the outer to inner ring, which may have important implications for biological light-harvesting function.

Mentions:
The differences between the molecular structures of the two chlorophyll species studied in this work are only in terms of side-groups, as shown in Fig. 1 where compared to ChlA, ChlB has a carbonyl oxygen (O71) in R7 substituent group. Yet they lead to noticeable differences in the absorption spectra and calculated rates of internal conversion. (See Figs 4 and 5 and Table 1.) To determine how these structural motifs influence the excited state dynamics, we analyzed the localization of the excited state wavefunction on specific atoms, functional groups, and regions of the molecule in terms of their time evolution as the system relaxes from the B → Qx → Qy bands in each of the chlorophyll species. Figure 6(a–d) shows the time evolution of averaged and normalized transition density localized on some important individual atoms for each of the chlorophyll species during the excited-state dynamics. In addition, the summed transition density over certain groups of atoms is depicted in Fig. 6(e,f). The total carbon macrocycle is defined as the carbon atoms comprising the porphyrin ring structure (shown as highlighted atoms in Fig. 1 excluding the N atoms), whereas the “inner macrocycle” (denoted by the dotted line in Fig. 1 excluding the N atoms) consists of atoms C1, C4, C5, C6, C9, C10, C11, C14, C15, C16, C19, and C20, and the “outer macrocycle” is comprised of carbon atoms from the total carbon macrocycle minus those in the inner macrocycle.

Mentions:
The differences between the molecular structures of the two chlorophyll species studied in this work are only in terms of side-groups, as shown in Fig. 1 where compared to ChlA, ChlB has a carbonyl oxygen (O71) in R7 substituent group. Yet they lead to noticeable differences in the absorption spectra and calculated rates of internal conversion. (See Figs 4 and 5 and Table 1.) To determine how these structural motifs influence the excited state dynamics, we analyzed the localization of the excited state wavefunction on specific atoms, functional groups, and regions of the molecule in terms of their time evolution as the system relaxes from the B → Qx → Qy bands in each of the chlorophyll species. Figure 6(a–d) shows the time evolution of averaged and normalized transition density localized on some important individual atoms for each of the chlorophyll species during the excited-state dynamics. In addition, the summed transition density over certain groups of atoms is depicted in Fig. 6(e,f). The total carbon macrocycle is defined as the carbon atoms comprising the porphyrin ring structure (shown as highlighted atoms in Fig. 1 excluding the N atoms), whereas the “inner macrocycle” (denoted by the dotted line in Fig. 1 excluding the N atoms) consists of atoms C1, C4, C5, C6, C9, C10, C11, C14, C15, C16, C19, and C20, and the “outer macrocycle” is comprised of carbon atoms from the total carbon macrocycle minus those in the inner macrocycle.

Bottom Line:
Nonradiative relaxation of high-energy excited states to the lowest excited state in chlorophylls marks the first step in the process of photosynthesis.Modeling this process with non-adiabatic excited state molecular dynamics simulations uncovers a critical role played by the different side groups in the two molecules in governing the intramolecular redistribution of excited state wavefunction, leading, in turn, to different time-scales.This is achieved via selective participation of specific atomic groups and complex global migration of the wavefunction from the outer to inner ring, which may have important implications for biological light-harvesting function.

ABSTRACTNonradiative relaxation of high-energy excited states to the lowest excited state in chlorophylls marks the first step in the process of photosynthesis. We perform ultrafast transient absorption spectroscopy measurements, that reveal this internal conversion dynamics to be slightly slower in chlorophyll B than in chlorophyll A. Modeling this process with non-adiabatic excited state molecular dynamics simulations uncovers a critical role played by the different side groups in the two molecules in governing the intramolecular redistribution of excited state wavefunction, leading, in turn, to different time-scales. Even given smaller electron-vibrational couplings compared to common organic conjugated chromophores, these molecules are able to efficiently dissipate about 1 eV of electronic energy into heat on the timescale of around 200 fs. This is achieved via selective participation of specific atomic groups and complex global migration of the wavefunction from the outer to inner ring, which may have important implications for biological light-harvesting function.